Note:
If I leave origin/master version of 1.txt, I can successfully perform commit after adding resolved file to index.

My guess that egit doesn't allow to perform merge commit if nothing is changed comparing to last local commit, which is my case as I'm leaving HEAD version. But I do need to perform merge commit, to be able to push to A.

So why I'm not able to perform merge commit, after leaving HEAD version of a file?